This invention relates to a servo system for a rotating member, and, more particularly, to a digital servo system for a rotating member.
With a video tape recorder, for example, a servo system including automatic frequency control (AFC) and automatic phase control (APC) is applied to control the rotation speed and phase of rotating members such as motors for driving a head disk and a capstan. With prior art video tape recorders, an analog servo system is generally used. According to a typical APC analog servo system for a head disk or head motor, a reference signal (a vertical synchronizing signal whose frequency is divided by a factor of two) has its waveform shaped to provide a sampling pulse. A rectangular rotation phase signal having the same frequency as the reference signal is formed of outputs from two disk tone heads spaced 180.degree. from each other and converted into a ramped trapezoidal signal. The ramp portion of the trapezoidal signal is sampled by the sampling pulse and held in a sample and hold circuit. The magnitude of the sampled signal thus held represents the momentary rotation phase of the head disk relative to the reference signal. The sampled analog signal in the sample and hold circuit is converted into a direct-current signal by a lowpass filter. The converted direct-current signal controls the head disk motor through a motor control amplifier.
The analog servo system so far described requires a sample and hold circuit necessitating a large capacitor and thus makes difficult integrated circuit version of a servo circuit. Moreover, parts used in the analog servo system are harmfully affected by secular changes in property as well as by variations in ambient temperature. The control loop of the analog servo system is prominently affected by external noises.